Coarse wavelength division multiplexing (CWDM) has recently emerged as an inexpensive technology for transporting multiple wavelength channels on a single optical fiber over moderate distances. CWDM's low cost relative to dense wavelength division multiplexing (DWDM) is attributed to the fact that the CWDM spectrum is orders of magnitude sparser than a typical DWDM spectrum. The ITU standard for CWDM defines a maximum of 18 wavelength channels with a channel-to-channel wavelength separation of 20 nm. That large channel spacing permits a 13-nm channel bandwidth, which in turn makes possible the use of inexpensive CWDM optics and directly modulated, un-cooled semiconductor laser transmitters. In contrast, DWDM systems, with typical channel spacings of 0.8 or 0.4 nm, require tightly specified and controlled laser transmitters, since the laser wavelength must fall within a small fraction of a nanometer over the entire life of the laser (typically ±0.1 nm for a system with 0.8-nm channel spacing). Their relatively small channel counts make CWDM systems the natural choice for transporting wavelengths at the edge of the network, where traffic is not highly aggregated as it is in the network core.
CWDM is considered an un-amplified technology since the large wavelength spread occupied by all channels in a typical commercial CWDM system (73 nm for a 4-channel system, 153 nm for an 8-channel system) cannot be accommodated by readily available low cost optical amplifiers. For example, inexpensive erbium-doped fiber amplifiers have an optical bandwidth of only about 30 nm. Being an un-amplified technology limits the reach of most commercial CWDM systems to approximately 80 km. That constraint could be overcome with the invention of a low-cost, broadband optical amplifier.
Although, in practice, semiconductor optical amplifiers (SOA) are capable of amplifying as many as 4 CWDM channels per SOA, the trade-off between maintaining sufficient optical signal-to-noise ratio (OSNR) and reducing gain saturation induced crosstalk reduces the dynamic range of pure SOA solutions while rendering them inadequate for systems with cascaded amplifiers.
Raman amplifiers have been tried in this application. A Raman amplifier is based on the nonlinear optical interaction between the optical signal and a high power pump laser. The gain medium may be the existing optical fiber or may be a custom highly non-linear fiber. A recently disclosed all-Raman amplifier covering the commercially-standard 8 CWDM channel wavelengths exhibited approximately 10 dB lower gain yet required 7 Raman pumps with widely varying pump powers, a total launched power over 1100 mW, and a custom highly nonlinear fiber (HNLF) gain medium.
Several fiber network providers are currently either evaluating or deploying CWDM systems to reduce costs. All those who deploy CWDM will have situations that require extending reach. With present technology, their only solution will be to install an expensive regenerator to perform the following steps: 1) optically demultiplex the CWDM channels; 2) convert each optical channel to analog electrical signals; 3) amplify the analog electrical signals; 4) recover the system clock; 5) use a decision circuit to regenerate a re-timed digital electrical data stream from the analog data and the recovered system clock; 6) use this electrical data to drive a CWDM laser transmitter for each channel; and 7) multiplex the various CWDM wavelengths onto the common transmission fiber. All of those (steps 1-7) could be replaced by a single low-cost optical amplifier.
There remains a need for a cost-effective amplifier that is useful with commercially-available CWDM systems, while minimizing the above-described disadvantages.